Date of Award

2022

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering (PhD)

Administrative Home Department

Department of Electrical and Computer Engineering

Advisor 1

Bruce A. Mork

Committee Member 1

Leonard J. Bohmann

Committee Member 2

Jeffrey B. Burl

Committee Member 3

R. Andrew Swartz

Abstract

When a transformer is subjected to a DC bias during a geomagnetic disturbance event (GMD), it leads to an effect known as half-cycle saturation. Due to this, reactive power losses increase in the system which may lead to voltage stability issues, and the transformer also starts to draw exciting current rich in harmonics.

In this dissertation, a study has been conducted on harmonic currents introduced due to geomagnetic induced current (GIC) for three-legged and five-legged transformers. The characteristics of harmonic currents depend on the type of transformer core and also air-path flux. This flux component with detailed core modeling using the hybrid model in ATPDraw of ATP-EMTP is considered. It is found that odd harmonics dominate in both the cores and harmonic content is more in five-legged compared to three-legged core. Moreover, as air-path inductance is increased, harmonic content in a transformer also increases. It is also shown that harmonics take a few tens of seconds to reach equilibrium during the GIC event.

In this work, harmonic currents introduced in a system due to GIC are also studied for the IEEE 20-bus GIC test system. In ATPDraw, a hybrid transformer model is used to model the system, and nonlinearities and frequency dependencies are considered. Time-domain simulations are conducted in ATP to analyze harmonics in transmission lines and a capacitor bank in a 20-bus system. This study has been performed for different geoelectric field (GEF) levels (EGEF) and orientations. Odd harmonic components in current are dominant in both transmission lines and a capacitor bank and harmonics take a few tens of seconds to reach equilibrium. High harmonic current or voltage may damage the capacitor bank and it may also trip during the GMD event.

Further, voltage stability analysis is performed on a 20-bus system in PowerWorld (phasor-domain approach) and ATP (time-domain approach). For this, QV curves are obtained for a base case without GEF (EGEF) and then with GEF. QV curves in ATPDraw are obtained at different time instants (60s, 190s) and it is shown that at 190s voltage stability is more deteriorated compared to 60s as all transformers are fully saturated at 190s. The advantage of the time-domain approach is that nonlinearities and frequency dependencies are included. These details are not included in PowerWorld. QV curves obtained using ATPDraw show that voltage stability is more deteriorated compared to PowerWorld QV curve during the GMD event.

In order to avoid a voltage collapse during a GMD event, emergency action strategies using a wide-area monitoring system (WAMS) are explained. For this, three voltage stability indices based on voltage magnitude, QV curve sensitivity, and rate of change of voltage are discussed. Then, real-time monitoring of harmonics using IEDs, calculation of voltage stability indices, situational awareness, and off-line simulations are combined to give a solution to a voltage collapse problem during a GMD event.

Available for download on Tuesday, April 18, 2023

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